Much of the molecular mechanism by which a light signal is transduced into an electrical signal in the eye has recently been discovered. A very important component of this system is the light-activated, cGMP phosphodiesterase that controls the concentration of cGMP in the photoreceptor outer segment. However, one of the areas that is least understood are the role(s) played by multiple isozymes of light-activated phosphodiesterases in this process. For example, it is known that different phosphodiesterases are present in different photoreceptor subtypes but not why they are there. The structural basis for probable differences in functional regulation of these isozymes is completely unknown. The long-term goals of this project are to help determine the molecular mechanisms responsible for the control of cyclic GMP metabolism and flux in mammalian photoreceptors. The first parts of the proposal address largely structural questions about the individual phosphodiesterase isozymes and make use of many of the newer techniques of microprotein sequencing. The last parts involve use of standard techniques of molecular biology to obtain and use a specific cDNA probes for each PDE subunit. They also utilize a Xenopus oocyte expression system to allow functional properties of the isolated subunits of the isozymes to be assessed and for domain organization to be determined. These approaches should complement each other and allow the structural basis for functional properties to be studied.